Gaseous emissions testing may include the testing of stack gas, stationary sources, mobile sources, ambient air modeling, or vehicle sources (such as engines or other power plants). For example, test sites for engine exhaust gas emission testing generally include one or more test benches. Each test bench includes multiple exhaust gas analyzers that determine concentrations of different gases in a sample of engine exhaust, which may be diluted or undiluted. The exhaust gas analyzers are periodically calibrated to ensure that the collected data is accurate.
The exhaust gas analyzers are connected to a source of non-reactive zero gas and a separate span gas for each exhaust gas analyzer range to be tested. The span gas supplies have a predetermined concentration of a particular gas, which the corresponding exhaust gas analyzer senses. A base reading is obtained from the exhaust gas analyzers by supplying only the zero gas. Intermediate readings are obtained for each exhaust gas analyzer by mixing the corresponding span gas and the zero gas and by supplying the mixture to the exhaust gas analyzer. Multiple tests are performed on each exhaust gas analyzer by varying the concentration of the span gas in the mixture. The process is repeated for the other exhaust gas analyzers on the test bench.
In one method, a gas divider dilutes the span gas using the zero gas to a first divide point. The divided gas is routed to the exhaust gas analyzer for a reading. The procedure is repeated for a discrete number of divide points. For example, ten divide points may be used. The process is performed for each of the exhaust gas analyzers on the test bench. The gathered data is used to create a best fit equation for each exhaust gas analyzer. The equations are programmed into a computer to calibrate the exhaust gas analyzers. The calibration process is time consuming and expensive due to the time required to allow the divisions of zero gas and span gas to stabilize. Additionally, extensive plumbing and control equipment is required.
Another method involves supplying a single blended span gas that includes all of the gases to which the exhaust gas analyzers on a test bench are responsive. The blended span gas is diluted with a zero gas and discrete divisions, for example 10 divisions, are supplied to all of the exhaust gas analyzers simultaneously for readings. The process is repeated for each exhaust gas analyzer range. When different exhaust gas analyzer ranges are used, drifting in the exhaust gas analyzers causes measurement errors. A discrete number of divisions of span gas and zero gas does not accurately describe an exhaust gas analyzer curve. Also, hysteresis in exhaust gas analyzers causes measurement errors.
Commercially available dividers can provide as many as 1000 divisions of span gas and zero gas. However, these dividers are very expensive. The amount of time that is required between divisions to allow the exhaust gas analyzer readings to stabilize is large.